Brushless electric motor systems have several failure modes, some of which are illustrated in FIG. 1. The illustrated motor has three stator phases 101, 102 and 103. Each phase is illustrated as a coil having only two turns such as 104 and 105. The phase coils pass through slots in a laminated iron stator pack 106. One end of each of the phase coils is connected to a common node at 107. The other ends 108, 109 and 110 of the phase coils are connected to a three phase bridge having six power switches 111, . . . , 116.
The fast switching of the transistors give high voltage ringing transients in the phase coils. This creates high local voltages specially where the phase windings cross each other on the way from one slot to another, and may create short circuit paths as illustrated at 117. The insulation against ground can also break as illustrated at 118. The insulation within a coil can fail as indicated at 119, creating a short circuit loop illustrated as a thicker line at 105. The transistors can fail to operate and then are in a permanent closed or open configuration.
All these failure modes will completely disable the operation of a conventional motor system.